In a variant of the oxo synthesis, which has been carried out on a large scale in industry for many years, the reaction of olefins with synthesis gas takes place in the presence of an aqueous solution of complex rhodium compounds as the catalyst. The principles of this process are described in DE-C-2 627 354, and its technical configuration is the subject of EP-B-103 810. A characteristic of this process is the use of rhodium compounds containing sulfonated triarylphosphines as complex ligands. Its advantages over the reaction using a homogeneous catalyst are based primarily on the formation of a very high proportion of unbranched aldehydes from n-olefins and the easy removal of the catalyst from the reaction product after the reaction has ended. This removal is performed simply by separating the aqueous phase from the organic phase, i.e. without distillation and thus without additional thermal process steps which lead to an impairment of the aldehyde yield. In addition to sulfonated triarylphosphines, the complex constituents of water-soluble rhodium complex compounds which are employed also include carboxylated triarylphosphines.
The two-phase process is outstandingly suitable for the hydroformylation of lower olefins, in particular ethylene and propylene. If higher olefins such as hexene, octene or decene are employed, the conversion is substantially reduced, so that synthesis on an industrial scale is no longer economic. This reduction in the conversion is caused by the decrease in solubility of higher olefins in water, since the reaction takes place in the aqueous phase.
One way of overcoming this disadvantage is the process disclosed in EP-B-157 316. In the hydroformylation of olefins having more than 6 carbon atoms in the presence of rhodium complex compounds containing trisulfonated triarylphosphines as the catalyst, the process consists of adding a quaternary ammonium salt to the aqueous catalyst solution as a solubilizer. The ammonium ion of the salt contains a straight or branched chain alkyl, .omega.-hydroxyalkyl, alkoxy, or a substituted or unsubstituted aryl radical, having in each case from 6 to 25 carbon atoms; and 3 identical or different straight or branched chain alkyl or .omega.-hydroxyalkyl radicals having 1 to 4 carbon atoms.
A modification of this process is the subject of EP-B-163 234. This patent teaches the reaction of C.sub.6 - to C.sub.20 -olefins with hydrogen and carbon monoxide in the presence of rhodium and a sulfonated arylphosphine whose cation is a quaternary ammonium ion. The ammonium ion contains one alkyl or aralkyl radical having 7 to 18 carbon atoms, and 3 straight or branched chain alkyl radicals having from 1 to 4 carbon atoms.
The processes described for the hydroformylation of higher olefins have proven very highly suitable in practice. However, the effectiveness of the quaternary ammonium salts used as the solubilizer over long periods of time leaves something to be desired, since they have only limited stability to hydrolysis and thermal stress. The consequence of this characteristic is that the aqueous catalyst solutions can be used for a shorter time than solutions containing no solubilizer.